Method for acquiring and interpreting transient electromagnetic measurements

ABSTRACT

A method for interpreting transient electromagnetic survey data includes measuring transient response of a medium over a plurality of switching events. The measured transient response to a first one of the current switching events is modeled. Transient response to the model for at least one current switching event prior in time to the at least a first current switching event is calculated. The calculated transient response is summed with the first event measured response and the sum is compared to the electromagnetic survey measurements. The model is adjusted and the calculating summed transient responses is repeated until a difference between the summed calculated responses and the survey measurements falls below a selected threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of transientcontrolled-source electromagnetic conductivity measurement apparatus andmethods subsurface Earth formations. More specifically, the inventionrelates to methods for acquiring and interpreting controlled-sourceelectromagnetic measurements that account for so-called “run-on”effects. The invention can be used with but is not limited to marineelectromagnetic and borehole electromagnetic surveying or geosteering.

2. Background Art

Controlled source electromagnetic surveying includes imparting anelectric current or a magnetic field into subsurface Earth formations,through the sea floor in marine surveying or through the borehole fluidin borehole surveying, and measuring voltages and/or magnetic fieldsinduced in electrodes, antennas and/or magnetometers disposed near theEarth's surface or on the sea floor. The voltages and/or magnetic fieldsare induced in response to the electric current and/or magnetic fieldimparted into the Earth's subsurface.

Controlled source surveying known in the art typically includesimparting alternating electric current into the subsurface. Thealternating current has one or more selected frequencies. Such surveyingis known as frequency domain controlled source electromagnetic (f-CSEM)surveying. f-CSEM surveying techniques are described, for example, inSinha, M. C. Patel, P. D., Unsworth, M. J., Owen, T. R. E., andMacCormack, M. G . R., 1990, An active source electromagnetic soundingsystem for marine use, Marine Geophysical Research, 12, 29-68. Otherpublications which describe the physics of and the interpretation ofelectromagnetic subsurface surveying include: Edwards, R. N., Law, L.K., Wolfgram, P. A., Nobes, D. C., Bone, M. N., Trigg, D. F., andDeLaurier, J. M., 1985, First results of the MOSES experiment: Seasediment conductivity and thickness determination, Bute Inlet, BritishColumbia, by magnetometric offshore electrical sounding: Geophysics 50,No. 1, 153-160; Edwards, R. N., 1997, On the resource evaluation ofmarine gas hydrate deposits using the sea-floor transient electricdipole-dipole method: Geophysics, 62, No. 1, 63-74; Chave, A. D.,Constable, S. C. and Edwards, R. N., 1991, Electrical explorationmethods for the seafloor: Investigation in geophysics No 3,Electromagnetic methods in applied geophysics, vol. 2, application, partB, 931-966; and Cheesman, S. J., Edwards, R. N., and Chave, A. D., 1987,On the theory of sea-floor conductivity mapping using transientelectromagnetic systems: Geophysics, 52, No. 2, 204-217. Typicalborehole related applications are described in Strack (U.S. Pat. Nos.6,541,975 B2, 6,670,813, and 6,739,165) and Hanstein et al., (U.S. Pat.No. 6,891,376). The proposed methodology is not limited to suchapplications because the array is moving along the survey area.

Following are described several patent publications which describevarious aspects of electromagnetic subsurface Earth surveying. For themarine case, U.S. Pat. No. 5,770,945 issued to Constable describes amagnetotelluric (MT) system for sea floor petroleum exploration. Thedisclosed system includes a first waterproof pressure case containing aprocessor, AC-coupled magnetic field post-amplifiers and electric fieldamplifiers, a second waterproof pressure case containing an acousticnavigation/release system, four silver-silver chloride electrodesmounted on booms and at least two magnetic induction coil sensors. Theseelements are mounted together on a plastic and aluminum frame along withflotation devices and an anchor for deployment to the sea floor. Theacoustic navigation/release system serves to locate the measurementsystem by responding to acoustic “pings” generated by a ship-board unit,and receives a release command which initiates detachment from theanchor so that the buoyant package floats to the surface for recovery.The electrodes used to detect the electric field are configured asgrounded dipole antennas. Booms by which the electrodes are mounted ontoa frame are positioned in an X-shaped configuration to create twoorthogonal dipoles. The two orthogonal dipoles are used to measure thecomplete vector electric field. The magnetic field sensors aremulti-turn, Mu-metal core wire coils which detect magnetic fields withinthe frequency range typically used for land-based MT surveys. Themagnetic field coils are encased in waterproof pressure cases and areconnected to the logger package by high pressure waterproof cables. Thelogger unit includes amplifiers for amplifying the signals received fromthe various sensors, which signals are then provided to the processorwhich controls timing, logging, storing and power switching operations.Temporary and mass storage is provided within and/or peripherally to theprocessor.

U.S. Pat. No. 6,603,313 B1 issued to Srnka discloses a method forsurface estimation of reservoir properties, in which location of andaverage earth resistivities above, below, and horizontally adjacent tosubsurface geologic formations are first determined using geological andgeophysical data in the vicinity of the subsurface geologic formation.Then dimensions and probing frequency for an electromagnetic source aredetermined to substantially maximize transmitted vertical and horizontalelectric currents at the subsurface geologic formation, using thelocation and the average earth resistivities. Next, the electromagneticsource is activated at or near surface, approximately centered above thesubsurface geologic formation and a plurality of components ofelectromagnetic response is measured with a receiver array. Geometricaland electrical parameter constraints are determined, using thegeological and geophysical data. Finally, the electromagnetic responseis processed using the geometrical and electrical parameter constraintsto produce inverted vertical and horizontal resistivity depth images.Optionally, the inverted resistivity depth images may be combined withthe geological and geophysical data to estimate the reservoir fluid andshaliness properties.

U.S. Pat. No. 6,628,110 B1 issued to Eidesmo et al. discloses a methodfor determining the nature of a subterranean reservoir whose approximategeometry and location are known. The disclosed method includes: applyinga time varying electromagnetic field to the strata containing thereservoir; detecting the electromagnetic wave field response; andanalyzing the effects on the characteristics of the detected field thathave been caused by the reservoir, thereby determining the content ofthe reservoir, based on the analysis.

U.S. Pat. No. 6,541,975 B2 and U.S. Pat. No. 6,670,813 issued to Strackdisclose a system for generating an image of an Earth formationsurrounding a borehole penetrating the formation. Resistivity of theformation is measured using a DC measurement, and conductivity andresistivity of the formations is measured with a time domain signal orAC measurement. Acoustic velocity of the formation is also measured. TheDC resistivity measurement, the conductivity measurement made with atime domain electromagnetic signal, the resistivity measurement madewith a time domain electromagnetic signal and the acoustic velocitymeasurements are combined to generate the image of the Earth formation.

U.S. Pat. No. 6,739,165 issued to Strack discloses a method wheretransient electromagnetic measurement are performed with a receiver ortransmitter being placed in a borehole and the other being placed on thesurface. Either is being moved and images of fluid content changes ofthe reservoir are obtained.

International Patent Application Publication No. WO 0157555 A1 disclosesa system for detecting a subterranean reservoir or determining thenature of a subterranean reservoir whose position and geometry is knownfrom previous seismic surveys. An n electromagnetic field is applied bya transmitter on the seabed and is detected by antennae also on theseabed. A refracted wave component is sought in the wave field response,to determine the nature of any reservoir present.

International Patent Application Publication No. WO 03048812 A1discloses an electromagnetic survey method for surveying an areapreviously identified as potentially containing a subsea hydrocarbonreservoir. The method includes obtaining first and second survey datasets with an electromagnetic source aligned end-on and broadsiderelative to the same or different receivers. The invention also relatesto planning a survey using this method, and to analysis of survey datataken in combination allow the galvanic contribution to the signalscollected at the receiver to be contrasted with the inductive effects,and the effects of signal attenuation, which are highly dependent onlocal properties of the rock formation, overlying water and air at thesurvey area This is very important to the success of usingelectromagnetic surveying for identifying hydrocarbon reserves anddistinguishing them from other classes of structure.

U.S. Pat. No. 6,842,006 B1 issued to Conti et al. discloses a sea-floorelectromagnetic measurement device for obtaining underwatermagnetotelluric (MT) measurements of earth formations. The deviceincludes a central structure with arms pivotally attached thereto. Thepivoting arms enable easy deployment and storage of the device.Electrodes and magnetometers are attached to each arm for measuringelectric and magnetic fields respectively, the magnetometers beingdistant from the central structure such that magnetic fields presenttherein are not sensed. A method for undertaking sea floor measurementsincludes measuring electric fields at a distance from the structure andmeasuring magnetic fields at the same location.

U.S. Patent Application Publication No. 2004 232917 relates to a methodof mapping subsurface resistivity contrasts by making multichanneltransient electromagnetic (MTEM) measurements on or near the Earth'ssurface using at least one source, receiving means for measuring thesystem response and at least one receiver for measuring the resultantearth response. All signals from the or each source-receiver pair areprocessed to recover the corresponding electromagnetic impulse responseof the earth and such impulse responses, or any transformation of suchimpulse responses, are displayed to create a subsurface representationof resistivity contrasts. The system and method enable subsurface fluiddeposits to be located and identified and the movement of such fluids tobe monitored.

U.S. Pat. No. 5,467,018 issued to Rueter et al. discloses a bedrockexploration system. The system includes transients generated as suddenchanges in a transmission stream, which are transmitted into the Earth'ssubsurface by a transmitter. The induced electric currents thus producedare measured by several receiver units. The measured values from thereceiver units are passed to a central unit. The measured valuesobtained from the receiver units are digitized and stored at themeasurement points, and the central unit is linked with the measurementpoints by a telemetry link. By means of the telemetry link, data fromthe data stores in the receiver units can be successively passed on tothe central unit.

U.S. Pat. No. 5,563,913 issued to Tasci et al. discloses a method andapparatus used in providing resistivity measurement data of asedimentary subsurface. The data are used for developing and mapping anenhanced anomalous resistivity pattern. The enhanced subsurfaceresistivity pattern is associated with and an aid for finding oil and/orgas traps at various depths down to a basement of the sedimentarysubsurface. The apparatus is disposed on a ground surface and includesan electric generator connected to a transmitter with a length of wirewith grounded electrodes. When large amplitude, long period, squarewaves of current are sent from a transmission site through thetransmitter and wire, secondary eddy currents are induced in thesubsurface. The eddy currents induce magnetic field changes in thesubsurface which can be measured at the surface of the earth with amagnetometer or induction coil. The magnetic field changes are receivedand recorded as time varying voltages at each sounding site. Informationon resistivity variations of the subsurface formations is deduced fromthe amplitude and shape of the measured magnetic field signals plottedas a function of time after applying appropriate mathematical equations.The sounding sites are arranged in a plot-like manner to ensure thatareal contour maps and cross sections of the resistivity variations ofthe subsurface formations can be prepared.

A limitation to f-CSEM techniques known in the art is that in marinesurveying they are typically limited to relatively great water depth, onthe order of 800-1,000 meters, or a ratio of ocean water depth tosubsurface reservoir depth (reservoir depth measured from the sea floor)of greater than about 1.5 to 2.0.

A typical f-CSEM marine survey can be described as follows. A recordingvessel includes cables which connect to electrodes disposed on the seafloor. An electric power source on the vessel charges the electrodessuch that a selected magnitude of current flows through the sea floorand into the Earth formations below the sea floor. At a selecteddistance (“offset”) from the source electrodes, receiver electrodes aredisposed on the sea floor and are coupled to a voltage measuringcircuit, which may be disposed on the vessel or a different vessel. Thevoltages imparted into the receiver electrodes are then analyzed toinfer the structure and electrical properties of the Earth formations inthe subsurface.

Another technique for electromagnetic surveying of subsurface Earthformations known in the art is transient controlled sourceelectromagnetic surveying (t-CSEM). In t-CSEM, electric current isimparted into the Earth at the Earth's surface, in a manner similar tof-CSEM. The electric current may be direct current (DC). At a selectedtime, the electric current is switched off, and induced voltages and/ormagnetic fields are measured, typically with respect to time over aselected time interval, at the Earth's surface. Structure of thesubsurface is inferred by the time distribution of the induced voltagesand/or magnetic fields. t-CSEM techniques are described, for example, inStrack, K.-M., 1992, Exploration with deep transient electromagnetics,Elsevier, 373 pp. (reprinted 1999).

A particular consideration in using t-CSEM surveying techniques is thatthe measured voltages and/or magnetic fields are as a practical matterrelated not only to the current switching event that the measurementsdirectly follow, but earlier current switching events. The amplitude ofthe measured magnetic fields and/or the measured voltages decays as timefrom the switching event increases. After a sufficient amount of timehas passed from a particular switching event, the amplitude of themeasured magnetic field and/or voltages decays substantially to zero.The amount of time required for the amplitudes to sufficiently decay mayin some instances be so long as to make it impractical to acquiremeasurements that do not have effect of prior switching events in themeasured magnetic fields and/or voltages. What is needed is a method toacquire t-CSEM measurements that takes account of prior currentswitching events so as to practically minimize the time betweensuccessive switching.

SUMMARY OF THE INVENTION

One aspect of the invention is a method for interpreting transientelectromagnetic survey data. A method according to this aspect of theinvention includes measuring transient response of a medium over aplurality of switching events. The measured transient response to afirst one of the current switching events is modeled. Transient responseto the model for at least one current switching event prior in time tothe at least a first current switching event is calculated. Thecalculated transient response is summed with the first event measuredresponse and the sum is compared to the electromagnetic surveymeasurements. The model is adjusted and the calculating summed transientresponses is repeated until a difference between the summed calculatedresponses and the survey measurements falls below a selected threshold.

Another aspect of the invention is a method for interpreting transientcontrolled source electromagnetic survey data. A method according tothis aspect of the invention includes generating an initial model ofconductivity distribution for a selected volume of the Earth'ssubsurface. Transient electromagnetic response of the initial model iscalculated for at least a first current switching event corresponding totransient controlled source electromagnetic survey measurements madeover the selected volume. Transient response to the initial model iscalculated for at least one current switching event prior in time to theat least a first current switching event. The calculated transientresponses are summed and, in some embodiments, a sufficient number ofprior switching events are included such that the effect of switchingevents prior to the at least a first switching event substantially nolonger contribute to the measurement. The summed transient responses arecompared to the electromagnetic survey measurements. The model isadjusted, and the calculating transient responses is repeated until adifference between the summed calculated responses and the surveymeasurements falls below a selected threshold.

A method for acquiring transient electromagnetic survey data accordingto another aspect of the invention includes inducing at least one of atransient magnetic field and a transient electric field in a selectedvolume in the Earth's subsurface. The inducing comprises at least afirst event of switching an electric current. At least one of a voltageand a magnetic field amplitude induced in response to the at least oneof transient magnetic field and transient electric field is detected. Aninitial model of conductivity distribution for the selected volume isgenerated. Transient electromagnetic response of the initial model forat least the first current switching event is calculated. Transientresponse to the initial model for at least one current switching eventprior in time to the at least a first current switching event iscalculated. The calculated transient responses are summed. The summedtransient responses are compared to the detected at least one ofmagnetic field amplitude and voltage, and the model is adjusted and thecalculating transient responses is repeated until a difference betweenthe summed calculated responses and the detected at least one ofmagnetic field amplitude and voltage falls below a selected threshold.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a marine transient electromagnetic survey system using ahorizontal electric dipole current source and a seismic source.

FIG. 1B shows a marine transient electromagnetic survey system using avertical electric dipole current source.

FIG. 2 shows an alternative way to energize the Earth using magneticfields.

FIG. 3 is a flow chart of one embodiment of a method according to theinvention.

FIG. 4 is a graph of induced voltage after current switching todemonstrate the effect of run on.

DETAILED DESCRIPTION

FIG. 1A shows one embodiment of a marine transient controlled sourceelectromagnetic (t-CSEM) survey system for use with methods according tovarious aspects of the invention. The system includes a survey vessel 10that moves in a predetermined pattern along the surface of a body ofwater 11 such as a lake or the ocean. The vessel 10 includes thereonsource actuation, recording and navigation equipment, shown generally at12 and referred to herein as the “recording system.” The recordingsystem 12 includes a controllable source of electric current used toenergize electrodes 16A 16B towed in the water 11 near the bottom 13thereof to impart an electric field in subsurface formations 15, 17below the bottom 13 of the water.). The recording system 12 includesinstrumentation to determine the geodetic position of the vessel 10 atany time, such as can be performed using global positioning system (GPS)receivers or the like. The recording system 12 includes equipment totransfer signals from one or more recording buoys 22. The recordingbuoys 22 receive and store signals from each of a plurality of t-CSEMsensors 20 positioned on the water bottom 13. The sensors may bedisposed along a cable 18. The cable 18 may be of a type used inconnection with seismic sensors deployed on the water bottom known inthe art as “ocean bottom cables.”

The sensors 20 detect various electric and/or magnetic fields thatresult from electric fields induced in the Earth's subsurface by currentpassing through the electrodes 16A, 16B. The recording buoys 22 mayinclude telemetry devices (not shown separately) to transmit data fromthe received signals to the vessel 10, and/or may store the signalslocally for later interrogation by the recording system 12 or by anotherinterrogation device.

The current source (not shown separately) on the vessel 10 is coupled tothe electrodes 16A, 16B by a cable 14A. The cable 14A is configured suchthat the electrodes 16A, 16B can be towed essentially horizontally nearthe water bottom 13 as shown in FIG. 1A. In the present embodiment, theelectrodes can be spaced apart about 50 meters, and can be energizedsuch that about 1000 Amperes of current flows through the electrodes16A, 16B. This is an equivalent source moment to that generated intypical electromagnetic survey practice known in the art using a 100meter long transmitter dipole, and using 500 Amperes current. In eithercase the source moment can be about 5×10⁴ Ampere-meters. The electriccurrent used to energize the transmitter electrodes 16A, 16B can bedirect current (DC) switched off at a time index equal to zero. Itshould be understood, however, that switching DC off is only oneimplementation of electric current change that is operable to inducetransient electromagnetic effects. In other embodiments, the current maybe switched on, may be switched from one polarity to the other (bipolarswitching), or may be switched in a pseudo-random binary sequence (PRBS)or any hybrid derivative of such switching sequences. See, for example,Duncan, P. M., Hwang, A., Edwards, R. N., Bailey, R. C., and Garland, G.D., 1980, The development and applications of a wide bandelectromagnetic sounding system using pseudo-noise source. Geophysics,45, 1276-1296 for a description of PRBS switching.

The vessel may also tow a seismic source 9 for contemporaneous seismicand electromagnetic surveying. In such embodiments, the water bottomcable 18 may incldue seismic sensors 21 of any type known in the art.

In the present embodiment, as the current through the transmitterelectrodes 16A, 16B is switched, a time-indexed recording of electricand/or magnetic fields detected by the various sensors 20 is recorded,either in the recording buoys 22 and/or in the recording system 12,depending on the particular configuration of recording and/or telemetryequipment in the recording buoys 22 and in the recording system 12.

FIG. 1B shows an alternative implementation of signal generation andrecording, in which the transmitter electrodes 16A, 16B are arrangedsuch that they are oriented substantially vertically along a cable 14Bconfigured to cause the electrodes 16A, 16B to be oriented substantiallyvertically as shown in FIG. 1B. Energizing the electrodes 16A, 16B,detecting and recording signals is performed substantially as explainedabove with reference to FIG. 1A.

The embodiments of FIG. 1A and FIG. 1B use electric current applied toelectrodes to impart an electric field into the Earth's subsurface. Analternative to electric fields is to use magnetic fields, and such willbe explained with reference to FIG. 2. In FIG. 2, the vessel 10 tows acable 14C which is connected to two loop transmitters 17A and 17B. Thefirst loop transmitter 17A encloses and area perpendicular to the waterbottom 13. Periodically, the recording system 12 causes electric currentto flow through the first loop transmitter 17A. The current can be inany of the same forms as described with reference to FIG. 1A, includingswitched DC, PRBS, and alternating polarity DC. When the currentchanges, a transient magnetic field having dipole moment along directionM_(A) is imparted into the Earth. At the same or at different times,current is applied to the second loop transmitter 17B. The second looptransmitter may be in the form of a solenoid coil, having a magneticmoment along direction M_(B).

The foregoing embodiments have been explained in the context of marineelectromagnetic surveying. It should be clearly understood that theforegoing embodiments are equally applicable to surveys conducted onland at the surface of the Earth or in a borehole. When conducted onland at the surface of the Earth, the sensors can be deployed insubstantially similar patterns to that shown in FIG. 1A. The surveycurrent source may be applied in the form of electric current, as shownin FIG. 1A, at the Earth's surface, or in the form of magnetic fields,as shown in and described with reference to FIG. 2. For purposes ofdefining the scope of the invention, the various survey devices can besaid to be disposed at the top of an area of the Earth's subsurface tobe surveyed. The top of the Earth's subsurface will be at the bottom ofthe water in a marine survey, and at the surface of the Earth in a landbased survey, or on the top of a layer of floating ice where suchsurveys are to be conducted.

One embodiment of an acquisition and processing method according to theinvention is shown in the form of a flow chart in FIG. 3. Transientelectromagentic data may be acquired substantially as explained abovewith reference to FIGS. 1A, 1B and 2. At 30, an initial model of theconductivity distribution in the Earth's subsurface is made for a volumeof the Earth's subsurface, typically that corresponds to the acquisitiongeometry at the time t-CSEM measurements are made. The volume willdepend on, among other factors, the positions of the various electrodesand/or loop antennas used during measurement acquisition. The initialmodel is used, at 32, to generate an expected transient response(whether in voltage or magnetic field amplitude) with respect to timefor a first selected switching event. As previously explained, suchswitching event may be current switch on, current switch off or currentpolarity reversal. Current polarity reversal, in some embodiments, mayinclude a short duration intervening current switch off, depending onthe apparatus used to make the measurements. It will be appreciated bythose skilled in the art that forward modeling programs known in the artfor calculating transient response do not take account of any undecayedeffects of prior current switching events.

In the present embodiment, at 34, a transient response for a switchingevent prior in time to the first switching event in the acquisitionsequence is calculated, preferably using the same forward modelingprocedure used to calculate the transient response for the firstswitching event, and using the same initial model of conductivitydistribution. At 36, the calculated transient response of the priorswitching event is evaluated with respect to a selected threshold. Theselected threshold may be, for example, a predetermined fraction of thepeak amplitude of the transient response of the first switching event.The selected threshold may be a predetermined peak amplitude value. Ifthe peak amplitude of the calculated response of the prior switchingevent is below the threshold, at 40, the calculated responses of theprior event and the first switching event are summed. The threshold isselected such that the effect of a switching event having such transientresponse is believed to substantially not affect the measured responseof the first switching event.

If the calculated response for the prior event is above the selectedthreshold, then at 38 the data are examined for a switching event backin time from the prior switching event. A transient response for suchback in time switching event is calculated at 34, just as for the priorswitching event. The foregoing process is repeated for successivelyearlier switching events until the peak amplitude of the calculatedtransient response for such switching event is below the selectedthreshold. At such time, the calculated transient responses for all suchswitching events are summed, at 40. The summed response is compared, at41, to the voltage and/or magnetic field actually measured at the firstswitching event. At 42, if the difference between the summed calculatedresponses and the measured response exceeds a selected threshold, atleast one parameter of the initial model is adjusted, at 46, and theprocess is repeated from 32 to 42. Such adjustment of the model, andrepetition of the process continues until the difference between thecalculated response and the measured response is below the selectedthreshold, at 44, at which point the process is completed with respectto the first switching event.

The foregoing procedure may be repeated for measurements correspondingto other volumes in the Earth's subsurface until the user has determineconductivity distribution over a desired total volume of the Earth'ssubsurface.

Alternatively, the measurements made be analyzed without reference to amodel of the Earth's subsurface. In such alternative implementation,transient response of some portion of the Earth's subsurface or othermedium is measured during a plurality of switching events. The responsewill include decaying amplitude of measured induced voltage and/ormagnetic field. The transient response after a first one of theswitching events may then be modeled such as by curve fit or othermathematical representation, or by equivalent analog circuit analysis,for example. In the present embodiment, at 34, a transient response fora switching event prior in time to the first switching event in theacquisition sequence is calculated, preferably using the same modelingprocedure used to calculate the transient response for the firstswitching event. At 36, the calculated transient response of the priorswitching event is evaluated with respect to a selected threshold. Theselected threshold may be, for example, a predetermined fraction of thepeak amplitude of the transient response of the first switching event.The selected threshold may be a predetermined peak amplitude value. Ifthe peak amplitude of the calculated response of the prior switchingevent is below the threshold, at 40, the calculated responses of theprior event and the first switching event are summed. The threshold isselected such that the effect of a switching event having such transientresponse is believed to substantially not affect the measured responseof the first switching event.

If the calculated response for the prior switching event is above theselected threshold, then at 38 the data are examined for a switchingevent back in time from the prior switching event. A transient responsefor such back in time switching event is calculated at 34, just as forthe prior switching event. The foregoing process can be repeated forsuccessively earlier switching events until the peak amplitude of thecalculated transient response for such switching event is below theselected threshold. At such time, the calculated transient responses forall such switching events are summed, at 40. The summed response iscompared, at 41, to the voltage and/or magnetic field actually measuredat the first switching event. At 42, if the difference between thesummed calculated responses and the measured response exceeds a selectedthreshold, at least one parameter of the model is adjusted, at 46, andthe process is repeated from 32 to 42. Such adjustment of the model, andrepetition of the process continues until the difference between thecalculated response and the measured response is below the selectedthreshold, at 44, at which point the process is completed with respectto the first switching event.

An example of the run on effect on transient response is shown in agraph in FIG. 4. The graph in FIG. 4 is a plot of apparent formationresistivity with respect to current switching event time. The currentswitching event used to generate the graph in FIG. 4 is a polarityreversal. The uppermost curve, labeled No Run On, represents thetransient response and apparent resistivity calculated therefrom whereno run on correction was used. The other curves represent transientresponse for one run on cycle correction through four run on cyclescorrection, respectively. The reason the one run on cycle correctionappears to have the greatest effect is believed to be related to thetype of current switching event, which as previously stated is apolarity reversal. Other types of switching events may provide differentresults with respect to the number of run on cycles of correction.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A method for interpreting transient electromagnetic survey data,comprising: generating an initial model of conductivity distribution fora selected volume of the Earth's subsurface; calculating transientelectromagnetic response of the model for at least a first currentswitching event corresponding to transient electromagnetic surveymeasurements made over the selected volume; calculating transientresponse to the model for at least one current switching event prior intime to the at least a first current switching event; summing thecalculated transient responses; comparing the summed transient responsesto the electromagnetic survey measurements; adjusting the model andrepeating the calculating transient responses until a difference betweenthe summed calculated responses and the survey measurements falls belowa selected threshold; and at least one of storing and displaying thelast adjusted model.
 2. The method of claim 1 further comprising:comparing peak amplitude of the calculated transient response of the atleast one prior switching event to a selected threshold; if the peakamplitude exceeds a selected threshold, calculating a transient responsefor a switching event prior in time to the at least one prior switchingevent; and repeating the comparing peak amplitude and calculatingtransient response for successively prior in time switching events untilthe peak amplitude is below the selected threshold.
 3. The method ofclaim 2 further comprising: summing the calculated transient responsesfor all the switching events; comparing the summed transient responsesto the survey measurements; and adjusting the model and repeating thecalculating all the transient responses until a difference between thesummed calculated responses and the survey measurements falls below aselected threshold.
 4. The method of claim 1 wherein the switching eventcomprises switching direct current off.
 5. The method of claim 1 whereinthe switching event comprises switching direct current on.
 6. The methodof claim 1 wherein the switching event comprises reversing directcurrent polarity.
 7. The method of claim 1 wherein the electromagneticsurvey measurements comprise voltage measurements made in response toelectric current imparted into the Earth's subsurface.
 8. The method ofclaim 1 wherein the electromagnetic survey measurements comprise voltagemeasurements made in response to a magnetic field imparted into theEarth's subsurface.
 9. The method of claim 1 wherein the electromagneticsurvey measurements comprise magnetic field amplitude measurements madein response to electric current imparted into the Earth's subsurface.10. The method of claim 1 wherein the electromagnetic surveymeasurements comprise magnetic field amplitude measurements made inresponse to a magnetic field imparted into the Earth's subsurface.
 11. Amethod for acquiring transient electromagnetic survey data, comprising:inducing at least one of a transient magnetic field and a transientelectric field in a selected volume in the Earth's subsurface, theinducing comprising at least a first event of switching an electriccurrent; detecting at least one of a voltage and a magnetic fieldamplitude induced in response to the at least one of transient magneticfield and transient electric field; generating an initial model ofconductivity distribution for the selected volume; calculating transientelectromagnetic response of the initial model for the at least the firstcurrent switching event; calculating transient response to the initialmodel for at least one current switching event prior in time to the atleast a first current switching event; summing the calculated transientresponses; comparing the summed transient responses to the detected atleast one of magnetic field amplitude and voltage; adjusting the modeland repeating the calculating transient responses until a differencebetween the summed calculated responses and the detected at least one ofmagnetic field amplitude and voltage falls below a selected threshold;and at least one of storing and displaying the last adjusted model. 12.The method of claim 11 further comprising: comparing peak amplitude ofthe calculated transient response of the at least one prior switchingevent to a selected threshold; if the peak amplitude exceeds a selectedthreshold, calculating a transient response for a switching event priorin time to the at least one prior switching event; and repeating thecomparing peak amplitude and calculating transient response forsuccessively prior in time switching events until the peak amplitude isbelow the selected threshold.
 13. The method of claim 12 furthercomprising: summing the calculated transient responses for all theswitching events; comparing the summed transient responses to the surveymeasurements; and adjusting the model and repeating the calculating allthe transient responses until a difference between the summed calculatedresponses and the survey measurements falls below a selected threshold.14. The method of claim 11 wherein the switching event comprisesswitching direct current off.
 15. The method of claim 11 wherein theswitching event comprises switching direct current on.
 16. The method ofclaim 11 wherein the switching event comprises reversing direct currentpolarity.
 17. The method of claim 11 wherein the detecting comprisesmeasurements made in response to electric current imparted into theEarth's subsurface.
 18. The method of claim 11 wherein the detectingcomprises voltage measurements made in response to a magnetic fieldimparted into the Earth's subsurface.
 19. The method of claim 11 whereinthe detecting comprises magnetic field amplitude measurements made inresponse to electric current imparted into the Earth's subsurface. 20.The method of claim 11 wherein the detecting comprises magnetic fieldamplitude measurements made in response to a magnetic field impartedinto the Earth's subsurface.
 21. A method for interpreting transientelectromagnetic survey data, comprising: measuring transient response ofa medium over a plurality of current switching events; modeling themeasured transient response of a first one of the current switchingevents; calculating transient response to the model for at least onecurrent switching event prior in time to the at least a first currentswitching event; summing the modeled prior event response with the firstevent response; comparing the summed transient responses to theelectromagnetic survey measurements; adjusting the model and repeatingthe calculating transient responses until a difference between thesummed calculated responses and the survey measurements falls below aselected threshold and at least one of storing and displaying theadjusted model.
 22. The method of claim 21 further comprising: comparingpeak amplitude of the calculated transient response of the at least oneprior switching event to a selected threshold; if the peak amplitudeexceeds a selected threshold, calculating a transient response for aswitching event prior in time to the at least one prior switching event;and repeating the comparing peak amplitude and calculating transientresponse for successively prior in time switching events until the peakamplitude is below the selected threshold.
 23. The method of claim 22further comprising: summing the calculated transient responses for allthe switching events; comparing the summed transient responses to thesurvey measurements; and adjusting the model and repeating thecalculating all the transient responses until a difference between thesummed calculated responses and the survey measurements falls below aselected threshold.
 24. The method of claim 21 wherein the switchingevent comprises switching direct current off.
 25. The method of claim 21wherein the switching event comprises switching direct current on. 26.The method of claim 21 wherein the switching event comprises reversingdirect current polarity.
 27. The method of claim 21 wherein theelectromagnetic survey measurements comprise voltage measurements madein response to electric current imparted into the Earth's subsurface.28. The method of claim 21 wherein the electromagnetic surveymeasurements comprise voltage measurements made in response to amagnetic field imparted into the Earth's subsurface.
 29. The method ofclaim 21 wherein the electromagnetic survey measurements comprisemagnetic field amplitude measurements made in response to electriccurrent imparted into the Earth's subsurface.
 30. The method of claim 21wherein the electromagnetic survey measurements comprise magnetic fieldamplitude measurements made in response to a magnetic field impartedinto the Earth's subsurface.